The present invention generally relates to signal-to-noise (S/N) ratio improving circuits for color video signals, and more particularly to a S/N ratio improving circuit for a video signal, which improves the picture quality of a picture obtained by reproducing the color video signal, by improving the S/N ratio of a color burst signal in the color video signal.
Generally, a noise component is included in a video signal which is reproduced by a video signal magnetic recording and reproducing apparatus. Conventionally, various circuits have been proposed for reducing this noise component and improving the S/N ratio of the video signal.
As one of these various conventional circuits, there was a S/N ratio improving circuit which employed a recursive filter using a one-frame delay line, as will be described hereinafter in conjunction with the drawings. In the television video signal, the correlation of the picture information is considerably high between each of the frames of the television video signal. On the other hand, there generally is no correlation between the noise components included in the video signal. Accordingly, when the video signal is averaged in terms of a period corresponding to one frame, there will be hardly any change in the energy of the video signal component, and only the energy of the noise component will decrease. The conventional S/N ratio improving circuit used this principle. Hence, the conventional S/N ratio improving circuit was designed so that the video signal is applied to one input terminal of an adder and an output of this adder is supplied to the other input terminal thereof through a one-frame delay line, and a video signal which is improved of its S/N ratio was obtained through the output of this adder.
However, according to the above conventional circuit, a signal which is delayed by one frame is supplied again to the one-frame delay line through the adder so as to be delayed by one frame, and such an operation was repeatedly carried out. Thus, different coefficients were multiplied by the two signals which were to be added in the adder before these signals were added, and the coefficients were selected so that the signal which is delayed by one frame gradually became attenuated. However, in any event, a signal component which is delayed by one or more frames was included in the output signal of the adder, and this signal component appeared as an afterimage when reproduced. Accordingly, there was no problem when the information content of the video signal was related to a still picture, however, such was generally not the case, and the information content of the video signal was more frequently related to a moving picture. Therefore, the afterimage became particularly conspicuous when the movement in the moving picture was quick, and such an after-image was unpleasant to the viewer.
In addition, in the television video signal, the correlation between the picture information is considerably high between each of the horizontal scanning lines of the television video signal, and the so-called line correlation exists. Hence, one may consider designing the S/N ratio improving circuit in which the one-frame delay line in the conventional circuit described before is replaced by a one horizontal scanning period (1H) delay line. When such a 1H delay line is employed and the video signal is averaged in terms of 1H, it is possible to reduce only the energy of the noise component without reducing the energy of the video signal component, as in the case of the conventional circuit described before.
However, according to the S/N ratio improving circuit which one may consider designing, a signal component which is delayed by 1H or more horizontal scanning periods is included in the output signal of the adder. For this reason, if there is a sudden change in the information content along the vertical direction of the picture, there will be a run in the color picture from a point where the change occurs throughout several lines below that point. Although there is line correlation in the picture content, strictly speaking, the picture content is generally different for each line except for the case where the picture is entirely of the same color. Hence, even though the S/N ratio is improved by the S/N ratio improving circuit, there is a disadvantage in that the color will run towards the lower part of the picture. This run in the color will become worse if an attempt is made to improve the S/N ratio. On the other hand, if an attempt is made to reduce the run in the color, the S/N ratio cannot be improved considerably, and the reduction of the run in the color and the improvement of the S/N ratio cannot be satisfactorily realized simultaneously.
On the other hand, when a noise component exists in the color video signal, this noise exists in both the video information signal portion and the color burst signal portion of the color video signal. When the noise exists in the video information signal portion, degradation is introduced in the picture quality of the reproduced picture due to this noise itself. Further, when the noise exists in the color burst signal portion, the phase of the color burst signal is affected by this noise and becomes unstable. Thus, in this case where the noise exists in the color burst signal portion, the phase of an output signal of a burst control oscillator for producing a local subcarrier signal in a receiver becomes unstable. The phases of the burst signal and the chrominance signal must constantly be maintained to their correct phases, however, when there is change in the phase due to the noise as described above, degradation will be introduced in the picture quality of the reproduced picture.
Accordingly, the present inventor has noted that the picture quality of the reproduced picture can be improved considerably if the noise existing in the burst signal portion is reduced, even if the noise existing in the video information signal portion is not reduced considerably or not reduced at all. Moreover, unlike the video information, the color burst signal will not differ for each of the horizontal scanning periods, and is always constant. That is, correlation exists with respect to the color burst signal. The present inventor has effectively used these characteristics.